There is an animal undercoat comb that people have been using. Mine is "Bamboo" brand that I had for my Himalayan cat and then just used it on the mini. It sort-of worked but he had other issues as well.

I always give a cruddy looking coat a good bath in iodine-based shampoo, then an good anti-fungal horse shampoo and then a good conditioner. It works every time. You also just never know with the babies. I've been to many nice farms in AZ that can have an interesting fungus on those halter yearlings.

If all else fails I would clip him. Get rid of that nasty coat since it's hot anyway (and close to the longest day of the year when they start to shed the new coat) so he can start growing a new one.